In a chemical plant, a rupture disc broke following the pressure rise (23 bar) on a photochlorination reactor; 1.5 tonnes of hydrogen chloride (HCl, by-product), 2 tonnes of methyl chloride (CH3Cl, reagent), 1.5 tonnes of methylene chloride, 1 tonne of chloroform and less than 0.5 tonnes of carbon tetrachloride (end products) were released into the atmosphere. To notify residents and request they remain indoors, the operator activated the external emergency plan siren and informed the Prefecture 30 min later. Maritime and rail traffic was halted south of the site. Pollution measurements recorded in the vicinity revealed an HCl concentration of below 0.3 ppm. Neighbours noticed a slight odour, yet no other human consequences were identified. Some 100 fire-fighters were deployed to attend to individuals in discomfort. The external emergency alert was lifted 1 hour after being activated. The accident was due to overly slow initiation of the photochlorination reaction, leading to an excessive amount of reagent in the reactor (Cl2 concentration > 35%). In turn, this excess reagent induced an exothermic reaction, causing a very rapid pressure increase that activated all of the safety systems (with the safety servo-control at 16 bar stopping the additional flow of reagents, as rupture discs set at 19 bar were connected to a neutralisation tower, while those at 23 bar were linked to the atmosphere). The slowing of photochlorination (due to fouled UV lamps at the end of their useful life), despite the contribution of reagents (Cl2 and CH3Cl), caused an accumulation of unconsumed Cl2 both in the environment and at extraction points in the downstream purification unit (distillation for recovering HCl then CH3Cl, which was later recycled). Since these ancillary columns and pipes had not been designed to receive Cl2, ferric chloride formed by contact with their sidewalls was injected along with the recycled CH3Cl. Iron salts, which are reaction inhibitors, accelerated the increase in Cl2 concentration to a high enough level (35%) to trigger the exothermic reaction. Several remedial measures were adopted: introduction of reaction start-up default detection (servo-control of the line valve dispersing HCl), plus detection of missed reactions (alarm tied to the temperature differential between reactor and heat exchanger, etc.) and detection of Cl2 accumulation (Cl2 analysis in the laboratory, monitoring of lamp fouling, etc.).